Multiple beam lasing systems can be used in a variety of applications, one of which is in a printing system where rotating polygonal mirrors are used to scan the light beams across a photoreceptive surface. The use of multiple laser beams in a printing system provides the capability of producing more than one line of information at a time, thus enabling high pel resolution, for example, 480 pels per inch or higher while keeping practical speeds for the rotating polygonal mirror. Multiple beam devices provide other capabilities as well; that is, the multiple beams can be used to alter the shape of the effective writing spot by modulating the spots within the spot group or they can be used to modulate the amount of light provided at each pel position.
While the use of multiple beam lasing systems have significant advantages over a single beam lasing system, multiple beam systems require precise mechanical alignment to assure that a print position (pel location) is properly located from line to line, that is, that a pel written by one laser is properly aligned with a pel written by another laser. Multiple beam systems which employ laser arrays require that the lasing diodes be placed on the chip at close tolerance in order to get correct pel placement; nevertheless, there can be some physical misplacement of the semiconductor laser diodes within the chip making it desirable to provide a system which can utilize laser arrays with some laser spot variation. Additionally, laser array chips are typically tilted to provide correct beam alignment in the non-scan direction. However, the tilt creates an offset from beam to beam in the scan direction, thus creating pel placement problems. As used herein, the term mechanical misalignment includes: (1) physical variation in the placement of lasing spots on a laser array chip, (2) mechanical misalignment of a laser array chip, or of discrete lasing sources within a machine; and (3) the offset produced by tilting a laser array.
Still another problem with multiple beam lasing systems is variable electrical propagation delays resulting from the use of different laser drivers for each laser source. Even though the drivers are of the same design, the different components comprising the drivers create a variable delay in turning the beams on and off from one laser to another, thus creating pel placement variations. Since these variations can change with the temperature and age of the laser driver, pel placement errors change dynamically.
Electronic correction schemes in the prior art achieve multiple pel correction for misalignments due to a necessary tilting of a laser array. Some of these electronic schemes provide sub-pel accuracy. Such systems require nanosecond resolution when high resolution systems are contemplated, that is, 480 pels per inch or higher. Clock frequencies of 500.times.10.sup.6 Hz are required to resolve to two nanoseconds. Such a clock and the high speed logic utilizing such resolution are expensive.
It is an object of this invention to provide an electronic correction scheme for use with a multiple beam scanning lasing system to provide correction for electrical propagation delays as well as for mechanical misalignment errors.
It is also an object of this invention to provide a closed loop electronic correction scheme for such errors.
It is still another object of this invention to provide an electronic correction scheme utilizing relatively low clock frequencies, but providing correction resolution within nanoseconds.
It is another object of this invention to provide a correction scheme which is relatively insensitive to variations in the intensity of different laser beams and in variations in the size of laser spots from beam to beam.
It is still another object of this invention to provide an electronic correction scheme which is dynamically adaptive to temperature and/or age as they create dynamic variations in mechanical misalignment or electrical propagation delay.